Always exclude pg_* tables in PostgreSQL truncation #74

It's sometimes necessary to add pg_catalog explicitly to the search path (if you don't want it added implicitly at the beginning of the search path). It then gets picked up by current_schemas(false) which causes DatabaseCleaner to attempt truncation of system tables. I can work around it with a monkey patch but I don't think there is ever a reason to want to truncate system tables -- could these be always excluded?